1. Field of the Invention
The present invention relates to a purification process which is suitable both for methacrylic acid and for acrylic acid. Below, the term (meth)acrylic acid means methacrylic acid or acrylic acid.
2. Description of the Background
Acrylic acid is a key chemical. Owing to its very reactive double bond and the acid function, it is particularly suitable as a monomer for the preparation of polymers. The greater part of the amount of acrylic acid monomers produced is esterified prior to polymerization—to give, for example, adhesives, dispersions or finishes. Only the smaller part of the acrylic acid monomers produced is polymerized directly—to give, for example, superabsorbers. While monomers of high purity are generally required in the direct polymerization of acrylic acid, the requirements relating to the purity of the acrylic acid are not so high when it is esterified prior to polymerization.
It is generally known that acrylic acid can be prepared by gas-phase oxidation of propene with molecular oxygen under heterogeneous catalysis over solid catalysts at from 200 to 400° C., in two stages via acrolein. Here, oxidic multicomponent catalysts, for example based on oxides of the elements molybdenum, chromium, vanadium or tellurium, are used.
Several processes have been proposed for working up the gas mixture obtained in the catalytic gas-phase oxidation.
WO-A-9 801 415 discloses a process for the preparation of (meth)acrylic acid in which the gas mixture obtained by catalytic gas-phase oxidation is condensed and (meth)acrylic acid can be crystallized from the resulting aqueous solution without addition of assistants.
EP-B-0 616 998 describes a process for purifying acrylic acid by means of fractional crystallization, the acrylic acid being purified by a combination of dynamic and static crystallization in a plurality of stages by means of crystallization/melting cycles, and the residue of the dynamic crystallization being further concentrated by the static crystallization. The dynamic crystallization described is either a falling-film layer crystallization or a suspension crystallization.